Thermal detector with inter-digitated thin film electrodes and method

ABSTRACT

Thermal sensor (36) may include a thermally sensitive element (50), a first thin film electrode (52) and a second thin film electrode (54). The thermally sensitive element (50) may include a plurality of preferentially-ordered crystals. The first thin film electrode (52) may include a plurality of digits (53) in communication with the thermally sensitive element (50). The digits (53) of the first thin film electrode (52) may be in spaced relation with one another. The second thin film electrode (54) may include a plurality of digits (55) in communication with the thermally sensitive element (50) opposite the first thin film electrode (52). The digits (55) of the second thin film electrode (54) may be in spaced relation with one another and in spaced interposed relation with the digits (53) of the first thin film electrode (52).

This application claims priority under 35 USC § 119(e)(1) of provisionalapplication Ser. No. 60/024,907, filed Aug. 30, 1996.

RELATED APPLICATIONS

This application is related to copending U.S. patent application Ser.No. 08/924,488, filed Feb. 9, 1998 entitled "THIN FILM ELECTRODE ANDMETHOD" now abandoned; copending U.S. patent application Ser. No.08,919,821 filed Aug. 28, 1997, entitled "THERMAL DETECTOR WITHPREFERENTIALLY-ORDERED THERMALLY SENSITIVE ELEMENT AND METHOD";copending U.S. patent application Ser. No. 08,916,039, filed Aug. 21,1997 entitled "METAL ORGANIC DECOMPOSITION (MOD) METHOD OF FORMINGTHERMALLY SENSITIVE ELEMENT"; copending U.S. patent application Ser. No.08/919,654 filed Aug. 28, 1997, entitled "THERMAL DETECTOR WITHNUCLEATION ELEMENT AND METHOD"; copending U.S. patent application Ser.No. 08/920,261, filed Aug. 22, 1997 entitled "THERMAL DETECTOR WITHSTRESS-ALIGNED THERMALLY SENSITIVE ELEMENT AND METHOD"; copending U.S.patent application Ser. No. 08/910,687 filed Aug. 13, 1997, entitled"METHOD OF PREFERENTIALLY-ORDERING A THERMALLY SENSITIVE ELEMENT".

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to infrared or thermal imaging systems,and more particularly to thermal detectors with inter-digitated thinfilm electrodes and method of fabrication.

BACKGROUND OF THE INVENTION

Thermal imaging systems are often employed to detect fires, overheatingmachinery, planes, vehicles and people, and to control temperaturesensitive industrial processes. Thermal imaging systems generallyoperate by detecting the differences in thermal radiance of variousobjects in a scene and by displaying the differences as a visual imageof the scene.

The basic components of a thermal imaging system generally includeoptics for collecting and focusing thermal radiation from a scene, athermal detector having a plurality of thermal sensors for convertingthermal radiation to an electrical signal, and electronics foramplifying and processing the electrical signal into a visual display orfor storage in an appropriate medium. A chopper is often included in athermal imaging system to A.C. couple the detector to the scene. Thechopper produces a constant background radiance which provides areference signal. The electronic processing portion of the thermalimaging system will subtract the reference signal from the totalradiance signal to produce a signal with minimum background bias.

The thermal sensors of a thermal imaging system may be disposed in afocal plane array. The focal plane array and its associated thermalsensors are often coupled to an integrated circuit substrate with acorresponding array of contact pads and a thermal isolation structuredisposed between the focal plane array and the integrated circuitsubstrate. The thermal sensors define the respective picture elements orpixels of the resulting thermal image.

One type of thermal sensor includes a thermally sensitive element formedfrom pyroelectric material which exhibits a state of electricalpolarization and/or change in dielectric constant dependent upontemperature changes of the pyroelectric material in response to incidentinfrared radiation. A pair of thin film electrodes are generallydisposed on opposite sides of the pyroelectric material to act ascapacitive plates. In this arrangement, the pyroelectric material actsas a dielectric, or insulator, disposed between the capacitive plates.Accordingly, the electrodes are operable to measure the charge generatedby the pyroelectric material in response to changes in temperature. Aspreviously discussed, the charge, or electrical signal, may be amplifiedand processed into a visual display.

The starting place for fabricating a thermal sensor is typically a waferof silicon or other suitable material. The wafer may have a diameter ofabout 150 millimeters (6 inches) and an approximate thickness of 660microns (26 mils). The materials that form the thermal sensors may bedeposited on the wafer in layers and removed as necessary.

SUMMARY OF THE INVENTION

A problem with producing thermal sensors having a pyroelectric layer,however, is that the pyroelectric material tends to orient itself withthe elongated, or polar, axis of its crystals parallel to theelectrodes. This orientation of the crystals relative to the electrodesdegrades the electrical signal of the thermal sensor, and thus of thevisual display of the thermal detector.

Accordingly, a need has arisen in the art for a thermal detector havingan improved thermal sensor. The present invention provides a thermalsensor with inter-digitated thin film electrodes that substantiallyeliminate or reduce the disadvantages and problems associated with priorthermal sensors.

In accordance with the present invention, a thermal sensor may providean image representative of the amount of thermal radiation incident tothe thermal sensor. The thermal sensor may include a thermally sensitiveelement, a first thin film electrode and a second thin film electrode.The thermally sensitive element may include a plurality ofpreferentially-ordered crystals. The first thin film electrode mayinclude a plurality of digits in communication with the thermallysensitive element. The digits of the first thin film electrode may be inspaced relation with one another. The second thin film electrode mayinclude a plurality of digits in communication with the thermallysensitive element opposite the first thin film electrode. The digits ofthe second thin film electrode may be in spaced relation with oneanother and in spaced interposed relation with the digits of the firstthin film electrode.

In accordance with one embodiment of the present invention, the firstthin film electrode may be disposed adjacent to the thermally sensitiveelement. The digits of the first thin film electrode may besubstantially parallel to one another and extend from a first end of thethermally sensitive element toward a second end of the thermallysensitive element. In this embodiment, the second thin film electrodemay be disposed adjacent to the thermally sensitive element opposite thefirst thin film electrode. The digits of the second thin film electrodemay be substantially parallel to one another and extend from the secondend of the thermally sensitive element toward the first end of thethermally sensitive element.

In a particular embodiment of the present invention, the thermallysensitive element may comprise a pyroelectric material. The pyroelectricmaterial may be lead lanthanum zirconate titanate (PLZT).

Important technical advantages of the present invention includeproviding a thermal detector having an improved thermal sensor. Inparticular, the thermal sensor includes inter-digitated electrodes thatgenerate an electric field largely within the plane of the thermallysensitive element. This electric field coincides with the naturalorientation of many pyroelectric materials, which is with the elongated,or polar, axis of crystals within the plane of the material. Alignmentof the electric field with the orientation of the pyroelectric materialincreases the electrical signal of the thermal sensor. The improvedelectrical signal, in turn, improves the visual display of the thermaldetector.

Other technical advantages will be readily apparent to one skilled inthe art from the following figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing the components of one embodiment of athermal imaging system constructed in accordance with the presentinvention;

FIG. 2 is an isometric view of the thermal detector of FIG. 1, showing afocal plane array mounted to an integrated circuit substrate opposite athermal element;

FIG. 3 is a detailed isometric view of the focal plane array of FIG. 2,showing a matrix of thermal sensors;

FIG. 4 is a detailed isometric view of one of the thermal sensors ofFIG. 3, showing a thermally sensitive element disposed between a pair ofinter-digitated electrodes; and

FIGS. 5A-5C are a series of elevation views in section showing variousstages of fabrication of the thermal sensor of FIG. 4 in accordance withone embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the present invention and its advantagesare best understood by referring now in more detail to FIGS. 1-5 of thedrawings, in which like numerals refer to like parts throughout theseveral views. FIG. 1 shows a schematic block diagram of a thermalimaging system 12 constructed in accordance with the present invention.During operation, the thermal imaging system 12 detects, processes, anddisplays the heat image of a scene 14.

The thermal imaging system 12 may be especially useful when imaging bymeans of visual wavelengths is unavailable, such as in the dark or whenvision is impaired by smoke, dust, or other particles. In suchconditions, the thermal imaging system 12 may detect thermal radiationin the infrared window. The infrared window is a wavelength region inthe infrared spectrum where there is good transmission ofelectromagnetic radiation through the atmosphere. Typically, infrareddetectors sense infrared radiation in the spectral bands from 3 to 5microns (having an energy of 0.4 to 0.25 eV) and from 8 to 14 microns(having an energy of 0.16 to 0.09 eV). The 3-5 micron spectral band isgenerally termed the "near infrared band" while the 8 to 14 micronspectral band is termed the "far infrared band." Infrared radiationbetween the near and far infrared bands cannot normally be detected dueto atmospheric absorption of the same. The thermal imaging system 12,however, is also useful during the day and when vision by means of thevisual wavelengths is available. For example, the thermal imaging system12 may be used to detect fires, overheating machinery, planes, vehiclesand people, and to control temperature sensitive industrial processes.

As shown in FIG. 1, the thermal imaging system 12 may comprise a lensassembly 16 in optical communication with a thermal detector 18. Thelens assembly 16 focuses or directs thermal radiation emitted by thescene 14 onto the thermal detector 18. The lens assembly 16 may includeone or more lenses made of material that transmits thermal radiation,such as germanium. The design of the lens assembly 16 may be varieddepending on the particular use of the thermal imaging system 12. Forexample, the lens assembly 16 may have a constant or a variable F-numberand/or may be a single field of view or a zoom lens.

The thermal detector 18 may be cooled or uncooled. A cooled thermaldetector is operated at cryogenic temperatures such as at thetemperature of liquid nitrogen, to obtain the desired sensitivity tovariation in infrared radiation. In cases where an uncooled detector 18is used, a chopper 20 is often disposed between the lens assembly 16 andthe thermal detector 18. Preferably, the lens assembly 16, thermaldetector 18 and chopper 20 are contained within an associated housing(not shown). The thermal detector 18 may also be contained within avacuum environment or an environment of low thermal conductivity gas.

The chopper 20 may be controlled by a signal processor 22 toperiodically interrupt transmission of the thermal image to the thermaldetector 18. Various types of mechanical and/or electrical choppers 20may be satisfactorily used with the present invention. For example, thechopper 20 may be a rotating disk with openings that periodically blockand let pass infrared radiation.

The placement of the lens assembly 16 and the chopper 20 with respect tothermal detector 18 is accomplished using well known principles ofoptical design as applied to thermal imaging systems. As previouslydescribed, the lens assembly 16 focuses thermal radiation emitted by thescene 14 onto the thermal detector 18. The thermal detector 18translates the incoming thermal radiation into corresponding electricalsignals for processing.

The electrical signals of the thermal detector 18 may be passed to thesignal processor 22, which assembles electrical signals into videosignals for display. As previously described, the signal processor 22may also synchronize operation of the chopper 20. This synchronizationenables the signal processor 22 to subtractively process incomingthermal radiation to eliminate fixed background radiation. The output ofthe signal processor 22 is often a video signal that may be viewed,further processed, stored, or the like.

The video signal of the signal processor 22 may be viewed on a localmonitor 24 or fed to a remote monitor 26 for display. The local monitor24 may be an eye piece containing an electronic viewfinder, a cathoderay tube, or the like. Similarly, the remote monitor 26 may comprise anelectronic display, a cathode ray tube, such as a television, or othertype of device capable of displaying the video signal. The video signalmay also be saved to a storage medium 28 for later recall. The storagemedium 28 may be a compact disk, a hard disk drive, random accessmemory, or any other type of medium capable of storing electronic videosignals for later recall. Monitors and storage mediums are well known inthe art and therefore will not be further described herein.

Electrical power to operate the thermal imaging system 12 may beprovided by a power supply 30. The power supply 30 provides electricalpower directly to the chopper 20, the thermal detector 18, the signalprocessor 22, and to the local monitor 24. Electrical power may also beprovided to the lens assembly 16, when, for example, a motor is employedto zoom the lens assembly 16.

FIG. 2 is a detailed view of the thermal detector 18. The thermaldetector 18 may comprise a focal plane array 32 mounted to a substrate34. In one embodiment, the focal plane array 32 may include a number ofthermal sensors 36 arranged in a matrix. The quantity and location ofthe thermal sensors 36 depend upon the N by M configuration desired forthe focal plane array 32.

The configuration of the focal plane array 32 generally varies fordifferent types of thermal detectors 18. In a "staring" thermaldetector, for example, the entire thermal image is focused onto a largefocal plane array. By contrast, a "scanning" thermal detector uses amirror or similar means to sweep successive portions of the thermalimage across a small focal plane array. Usually, although not necessaryfor the invention, both types of thermal detectors 18 consist of anumber of thermal sensors 36, with the output of each thermal sensor 36representing a portion of the viewed scene 14. For example, the outputof each thermal sensor 36 in focal plane array 32 may represent a singlepixel of the total image. This embodiment may be particularly beneficialfor use in connection with high density visual displays.

The substrate 34 may be an integrated circuit substrate that providesthe necessary electrical couplings and circuitry to process the thermalimage formed on the focal plane array 32. The integrated circuitsubstrate 34 may be formed of silicon, ceramic alumina, or othersuitable materials which are both chemically and thermally compatiblewith the multiple layers which will be formed on the surface 66 (FIG. 4)of the integrated surface substrate 34. Further information regardingthermal sensors mounted on an underlying integrated circuit substrate isdisclosed by U.S. Pat. No. 4,143,269 issued to McCormack, et al.,entitled "Ferroelectric Imaging System" and U.S. Pat. No. 5,021,663issued to Hornbeck, entitled "Infrared Detector."

A thermal element 38 may be provided to maintain the integrated circuitsubstrate 34 at a constant predefined temperature. The constanttemperature prevents ambient or internally generated temperaturegradients from affecting the thermal sensors 36 and thus provides abaseline with which the thermal energy of the scene 14 can be accuratelymeasured. The necessary electrical couplings and circuitry to controlthe thermal element 38 may be provided by the integrated circuitsubstrate 34. In such a case, the thermal element 38 may be coupled tothe integrated circuit substrate 34 opposite the focal plane array 32.

FIG. 3 illustrates a detailed view of the previously described focalplane array 32. In this embodiment, the focal plane array 32 includes amatrix of thermal sensors 36. Each thermal sensor 36 forms a discreteelement of the focal plane array 32. The thermal sensors 36 may beseparated by a set of intersecting slots 40 provided around theperimeter of each thermal sensor 36. The slots 40 provide a high degreeof reticulation between adjacent thermal sensors 36 that substantiallyreduces thermal spreading between the pixel elements.

The thermal sensors 36 may detect thermal radiation using varioustechniques. For example, the thermal sensors may be based upon thegeneration of a charge due to a change in temperature resulting fromthermal radiation heating the thermal sensors 36. Alternatively, thethermal sensors 36 may be based upon the generation of a charge due to aphoton-electron interaction within the material used to form the thermalsensors 36. This latter effect is sometimes called the internalphotoelectric effect. The thermal sensors 36 may also be based upon thechange in resistance of a thin conductor caused by the heating effect ofthermal radiation. Such thermal sensors 36 are sometimes referred to asbolometers. It will be understood that these and other types of thermalsensors 36 may be used in accordance with the present invention.

FIG. 4 illustrates a detailed view of one of the thermal sensors 36.Each thermal sensor 36 may include a thermally sensitive element 50, afirst electrically conductive element 52 and a second electricallyconductive element 54. In one embodiment, the thermally sensitiveelement 50 is preferably formed from pyroelectric materials. Thepyroelectric materials may also be ferroelectric materials such asbarium strontium titanate (BST), barium titanate (BT), and antimonysulfoiodide (SbSI), or any lead containing ferroelectric materialincluding lead titanate (PT), lead lanthanum titanate (PLT), leadzirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), leadzinc niobate (PZN), lead strontium titanate (PSrT), and lead scandiumtantalate (PST). In this embodiment, the thermally sensitive element 50generates a charge in response to a change in temperature. It will beunderstood, however, that the present invention contemplates forming thethermally sensitive element 50 from any thermal sensitive material thatprovides a satisfactory response to thermal radiation.

The thickness of the thermally sensitive element 50 may vary dependingupon the wavelength of thermal radiation that the thermal imaging system12 is designed to detect. The thermally sensitive element 50 ispreferably a thin film to enhance responsiveness to thermal radiationand transmission of a generated charge to the electrically conductiveelements 52 and 54.

The thermally sensitive element 50 may comprise a plurality ofpreferentially-ordered crystals. The crystals may bepreferential-ordered naturally or may be preferentially-ordered in theprocess of fabricating the thermal sensor 36. Preferably, the polar axisof the crystals is within the plane of the thermally sensitive element50. As described in more detail below, this orientation coincides withthe electric field generated by the first and second electricallyconductive elements 52 and 54 of the present invention. Alignment of theelectrical field with the orientation of the pyroelectric materialincreases the electrical signal of the thermal sensor 36. The improvedelectrical signal, in turn, improves the visual display obtained fromthe thermal sensor 36.

The first electrically conductive element 52 and the second electricallyconductive element 54 may be disposed on the same side of the thermallysensitive, or pyroelectric, element 50. Alternately, the electricallyconductive elements may be disposed on opposite sides of thepyroelectric element 50. In both arrangements, the electricallyconductive elements 52 and 54 function as electrodes that receivecharges generated by the pyroelectric element 50 in response to thermalradiation. Accordingly, the electrodes 52 and 54 are in electricalcommunication with the pyroelectric element 50, which includescapacitive coupling.

In accordance with the present invention, the electrically conductiveelements, or electrodes, 52 and 54 may be inter-digitated electrodes.The first electrode 52 may include a plurality of digits 53 inelectrical communication with the thermally sensitive element 50.Preferably, the digits 53 are in space relation and parallel to oneanother. The second electrode 54 may include digits 55 in electricalcommunication with the thermally sensitive element 50 opposite the firstelectrode 52. Preferably, the digits 55 are in space relation andparallel to one another.

The digits 53 of the first electrode 52 may be in interposed relation tothe digits 55 of the second electrode 54. As a result of the interposedrelation, the electrodes 52 and 54 generate an electric field largelywithin the plane of the thermally sensitive element 50. This electricfield coincides with the natural crystallographic orientation of thethermally sensitive element 50, which is with the polar axis within theplane of the thermally sensitive element 50. Alignment of the electricfield with the crystals of the thermally sensitive element 50 increasesthe electrical signal of the thermal sensor 36, and thus improves thevisual display of the thermal detector 18.

In one embodiment, the first and second electrodes 52 and 54 may bedisposed on an upper surface of the pyroelectric layer 50. The digits 53of the first electrode 52 may extend from a first end of the thermallysensitive element 50 toward a second end of the thermally sensitiveelement 50. The digits 55 of the second electrode 54 may extend from thesecond end of the thermally sensitive element 50 toward the first end ofthe thermally sensitive element 50.

In an embodiment in which the electrodes are disposed on opposite sidesof the thermally sensitive element 50, the first electrode 52 may bedisposed adjacent to the thermally sensitive element 50. In thisembodiment, the second electrode 54 may be disposed adjacent to thethermally sensitive element 50 opposite the first electrode 52. Thedigits 53 of the first electrode 52 may extend from a first end of thethermally sensitive element 50 toward a second end of the thermallysensitive element 50. Preferably, the digits 53 extend completely fromthe first end to the second end of the thermally sensitive element 50.The digits 55 of the second electrode 54 may extend from the second endof the thermally sensitive element 50 toward the first end of thethermally sensitive element 50. Preferably, the digits 55 extendcompletely from the second end to the first end of the thermallysensitive element 50.

The electrically conductive electrodes 52 and 54 may be thin filmelectrodes. Thin film electrodes 52 and 54 are generally preferredbecause they may be virtually transparent to thermal radiation. Thinfilm electrodes are also preferred because they do not rob absorbedthermal energy from the pyroelectric element 50.

The electrodes 52 and 54 may be constructed of a solid solution ofplatinum and rhodium as described in related U.S. patent applicationSer. No. 08/924,488 filed Feb. 9, 1998, entitled "THIN FILM ELECTRODEAND METHOD." Alternately, the electrodes 52 and 54 may be formed ofvarious single component materials that are electrically conductive. Forexample, the second electrode 54 may be formed of palladium or platinum,or from conductive oxides such as ruthenium oxide (RuO₂) or lantbanumstrontium cobalt oxide (LSCO).

The thermal sensor 36 is preferably self-supported above the integratedcircuit substrate 34. As shown by FIG. 4, a first support arm 56preferably extends from the first electrode 52. A second support arm 58preferably extends from the second electrode 54. In another embodiment,the thermally sensitive element 50 may be divided into separate sectionsand the support arms 56 and 58 may extend from a bifurcated electrode 52or 54.

For many applications, the support arm 56 is preferably formed from thesame type of material as the first electrode 52. Similarly, the supportarm 58 is preferably formed from the same type of material as the secondelectrode 54. The support arms 56 and 58 however may be formed of adifferent material than the electrodes 52 and 54. Additionally, thethickness of the support arms 56 and 58 may be varied to control thermalconductance between the electrodes and the integrated circuit substrate34. Thermally sensitive material may be disposed above the support arm56 and below the support arm 58.

The length, width and thickness of the support arms 56 and 58 may beselected to enhance their resistance to the transfer of thermal energybetween the thermal sensor 36 and the integrated circuit substrate 34.In one embodiment, for example, slots 60 and 62 may be formed betweeneach support arm and its respective electrode to bifurcate the supportarms and thus provide additional thermal isolation between the supportarms and their associated electrodes. In this embodiment, the thermalisolation of each support arm may be increased by lengthening thebifurcated support arm. Thermal isolation may be maximized by fullyextending each support arm along opposite halves of the perimeter of theelectrodes.

A pair of posts 64 may be provided to support the bifurcated supportarms 56 and 58, and thus the thermal sensor 36, in spaced relation witha surface 66 of the integrated circuit substrate 34. The posts 64 mayeach support one of the bifurcated support arms. The posts 64 arepreferably formed from material which is electrically conductive. Inthis embodiment, each post 64 may transmit electrical signals from itsrespective electrode to a contact pad 70 of the integrated circuitsubstrate 34. Thus, the posts 64 provide both mechanical support and asignal flowpath to the associated contact pad 70.

A chamber 68 may be formed by the gap between the bottom of thepyroelectric element 50 and the surface 66 of the integrated circuitsubstrate 34. The pyroelectric element 50 may absorb thermal radiationdirectly or in part after the radiation has passed through the chamber68 and reflected off the integrated circuit substrate 34. For anembodiment in which thermal radiation is absorbed in part afterreflecting off the integrated circuit substrate 34, the dimensions ofthe chamber 68 may be varied depending upon the wavelength of thethermal radiation that the thermal imaging system 12 is designed todetect. The chamber 68 preferably corresponds to about one-fourth of theselected thermal radiation wavelength. Thus, if thermal imaging system12 is designed to detect thermal radiation having a wavelength of 7.5 to14 microns, the chamber 68 preferably has a height of approximately twoto three microns. In this embodiment, the electrodes 52 and 54 may betransparent to thermal radiation. The ability to vary the position ofbottom of the thermally sensitive element 50 with respect to the surface68 of the integrated substrate 34 enhances the responsiveness of thermalsensor 36 to thermal radiation.

FIGS. 5A-C depict various steps during the process of fabricating thethermal sensors 36 in accordance with an embodiment of the presentinvention. As shown in FIG. 5A, an array of contact pads 70 may bedisposed on the surface 66 of the integrated circuit substrate 34 toreceive electrical signals generated by the thermal sensors 36. Aspreviously described, the integrated circuit substrate 34 may be formedfrom silicon or other suitable materials which are both chemically andthermally compatible with the multiple layers which will be formed onthe surface 66 of the integrated surface substrate 34.

A sacrificial layer 72 may be deposited on the integrated circuitsubstrate 34. During the fabrication process, the sacrificial layer 72forms a base upon which the thermal sensors 36 may be formed in spacerelation with the integrated circuit substrate 34. Accordingly, thesacrificial layer 72 may be removed after processing to yield thechamber 68.

The sacrificial layer should have a thickness equal to the desiredheight of the chamber 68. As previously described, the height of thechamber preferably corresponds to one-fourth of the selected thermalradiation wavelength. Thus, if thermal sensors 36 are to detect thermalradiation having a wavelength of 7.5 to 14 microns, the sacrificiallayer should be deposited in a thickness of approximately two to threemicrons. The sacrificial layer 72 is preferably silicon dioxide (SiO₂)or polyimide or a similar type of material compatible with fabricationof the thermal sensors 36. A material is compatible with the fabricationof the thermal sensors 36 when it will not inordinately shrink or expandor burn, melt or interact with other materials to an extent that itinterferes with processing. The material of the sacrificial layer 72 isalso preferably removable by way of dry etching techniques.

A layer of thermally sensitive material 80 may be formed on thesacrificial layer 72, as shown in FIG. 5A. As described below, the layerof thermally sensitive material 80 will form the thermally sensitiveelement 50. In one embodiment, the layer of thermally sensitive material80 is preferably formed from pyroelectric material such as bariumstrontium titanate (BST), barium titanate (BT), and antimony sulfoiodide(SbSI). In other embodiments, lead containing ferroelectric materialsincluding lead titanate (PT), lead lanthanum titanate (PLT), leadzirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), leadzinc niobate (PZN), lead strontium titanate (PSrT), and lead scandiumtantalate (PST) may be used to form the thermally sensitive layer 80.The selection of material for the thermally sensitive layer 80 dependsupon the type of thermal sensor 36 which will be formed on theintegrated circuit substrate 34.

A layer of electrically conductive material 85 may next be formed on thethermally sensitive layer 80. As described below, the first layer ofelectrically conductive material 85 will form the first and secondelectrodes 52 and 54. It will be understood that the electrodes may beformed on a bottom surface of the thermally sensitive layer 80 or onopposite sides of the thermally sensitive layer 80. In theseembodiments, spaces formed between the digits 53 and/or 55 may be filledin with a filler material to form an even surface upon which thethermally sensitive material may be deposited. The filler material maybe the material of the sacrificial layer 72. The filler material betweenthe digits 53 and/or 55 may later be removed along with the sacrificiallayer 72.

The layer of electrically conductive material 85 may be formed ofvarious types of materials such as palladium or platinum. However, thepresent invention allows other types of electrically conductive materialto be used depending on the type of thermal sensors 36 which will beformed on the integrated circuit substrate 34. For example, aspreviously described, the layer of electrically conductive material 85may be constructed of a solid solution.

The layer of electrically conductive material 85 may be reticulated toform the digits 53 of the first electrode 52 and the digits 55 of thesecond electrode. Various photolithographic techniques includinganisotropic etching processes may be used to reticulate the layer ofelectrically conductive material 85 to define the digits 53 and 55.Depending on the type of material used to form the layer of electricallyconductive material 85, the anisotropic etching process may includeoxygen-based ion milling, reactive ion etch (RIE) or a magneticallyenhanced reactive ion etch (MERIE).

A barrier layer may be formed between the electrically conductive layer85 and the thermally sensitive layer 80. A barrier layer may be desired,when, for example, the material of the electrically conductive layer isnot compatible with the thermally sensitive material. In such a case,the electrically conductive layer may communicate with the thermallysensitive material through the barrier layer.

Various techniques may be used to form thin film layers 72, 80 and 85.Often these techniques are divided into two groups--film growth byinteraction of a vapor deposited species with an associated substrateand film formation by deposition without causing changes to theassociated substrate. The first group of thin film growth techniquesincludes thermal oxidation and nitridation of single crystal silicon andpolysilicon. The formation of silicides by direct reaction of adeposited metal and the substrate is also frequently included in thisfirst group of thin film growth techniques.

The second group of thin film growth techniques may be further dividedinto three subclasses of deposition. The first subclass is oftenreferred to as chemical vapor deposition (CVD) in which solid films areformed on a substrate by the chemical reaction of vapor phase chemicalswhich contain the desired constituents for the associated thin filmlayer. The second subclass is often referred to as physical vapordeposition (PVD) in which the desired thin film layer is physicallydislodged from a source to form a vapor and transport it across areduced pressure region to the substrate. The dislodged layer is thencondensed to form the desired thin film layer. The third subclasstypically involves coating the substrate with a liquid which is thendried to form the desired thin film layer. The formation of thin filmlayers by PVD includes such processes as sputtering, evaporation andmolecular beam epitaxy. Spin coating is one of the most commonly usedtechniques for depositing liquids on a substrate to form a thin filmlayer.

Thin film layers may also be satisfactorily grown in accordance with theteachings of the present invention by using techniques such as dipping,vapor phase deposition by sputtering or MOCVD, and sol/gel or metalorganic decomposition (MOD) by spin coating. Processes should beselected to establish the desired electrical and thermal characteristicsfor the resulting thermal sensors 36. Additionally, depending upon thetype of materials used to form layers 72, 80 and 85, one or more bufferlayers or protective layers (not shown) may be disposed between surface66 of integrated circuit substrate 34 and/or layers 72, 80 and 85.

The various techniques may be integrated to allow fabrication of thethermal sensors 36 on integrated circuit substrate 34 using processesassociated with the manufacture of very large scale integrated circuits.Material usage and overall process efficiency associated withfabricating a focal plane array 32 may be substantially improved. Forexample, thermally sensitive layer 80 is preferably formed withapproximately the same thickness as desired for thermal sensitiveelements 50. Thus, the possibility of polishing damage associated withprevious techniques used to form thermal sensitive elements frompyroelectric materials have been substantially reduced or eliminated.

As shown by FIG. 5B, a pair of vias 90 may be formed for each thermalsensor 36. The vias 90 are preferably formed using anisotropic etchingor other photolithographic techniques. The post 64 may be formed byfilling the vias 90 with a supporting material. For one application, thesupporting material 95 may be platinum. However, other types of materialmay be used for the post 64 depending on the type of thermal sensors 36that are to be fabricated, as well as the temperatures and processesinvolved in fabrication.

As shown by FIG. 5C, after forming the desired layers of material 80 and85 on the surface 66 of the integrated circuit substrate 34 and theposts 64 in the layers, individual thermal sensors 36 may be defined onthe integrated circuit substrate 34. As previously discussed, thesacrificial layer 72 is removed during processing to form the cavity 68.Any filler material between digits of the layer of electricallyconductive material may also be removed. Various photolithographictechniques including anisotropic etching processes may be used to definethe desired thermal sensors 36. Depending upon the type of materialsused to form layers 80 and 85, the anisotropic etching processes mayinclude oxygen-based ion milling, reactive ion etch (RIE) or amagnetically enhanced reactive ion etch (MERIE).

In the resulting thermal sensors 36, the thermally sensitive element 50is preferably formed from the thermally sensitive layer 80. The firstand second electrodes 52 and 54 are preferably formed from the secondlayer of electrically conductive material 85. The posts 64 are formedfrom the supporting material 95 in the vias 90. The posts 64 rest on thecontact pads 70.

Additionally, the thermal sensors 36 preferably include the cavity 68between the thermally sensitive element 50 and the surface 66 ofintegrated circuit substrate 34. As previously discussed, the cavity 68will preferably have a height which corresponds to about one quarter ofthe wavelength of the incident infrared radiation which will be detectedby thermal sensors 36. For one application, post 64 and the associatedcavity 68 have a height of approximately two and one-half microns.

After the individual thermal sensors 36 have been formed, an electricalpotential may be applied between the thermally sensitive element 50 andthe first and second electrodes 52 and 54 of each thermal sensor 36. Theelectrical potential may shift the polar axis of crystals that are notalready in the plane of the thermally sensitive element 50 to withinthat plane. As previously described, crystals of the thermally sensitiveelement 50 are naturally preferentially-ordered with their polar axis inthe plane of thermally sensitive element 50. Thus, the electric fieldincreases the number of crystals that have their polar axis in the planeof the thermally sensitive element 50.

The sole table below provides an overview of some embodiments in thedrawings.

    ______________________________________                                        Drawing                                                                              Preferred or                                                             Element Specific Term Generic Term Alternate Terms                          ______________________________________                                        16     Lens Assembly                                                                             Optics                                                       20 Chopper Device that                                                          interrupts beam                                                               of radiation                                                                22 Signal Processor Electronics                                               24 Monitor Display Electronic                                                    viewfinder, Cathode                                                           ray tube                                                                   32 Focal plane array Matrix of                                                  thermal sensors                                                             34 Integrated Silicon                                                          circuit substrate switching                                                    matrix                                                                      36 Pyroelectric Thermal sensors Bolometers                                     sensors                                                                      50 Pyroelectric Thermally Barium strontium                                     element sensitive titanate (BST),                                              element barium titanate                                                        (BT), and antimony                                                            sulfoiodide (SbSI),                                                           or any lead                                                                   containing                                                                    ferroelectric                                                                 material including                                                            lead titanate (PT),                                                           lead lanthanum                                                                titanate (PLT),                                                               lead zirconate                                                                titanate (PZT),                                                               lead lanthanum                                                                zirconate titanate                                                            (PLZT), lead zinc                                                             niobate (PZN), lead                                                           strontium titanate                                                            (PSrT), and lead                                                              scandium tantalate                                                            (PST).                                                                     52 First thin film First Solid solution,                                       electrode electrically platinum or                                             conductive palladium                                                          element                                                                     54 Second thin film Second Solid solution,                                     electrode electrically platinum or                                             conductive palladium                                                          element                                                                     53, 55 Digits Extensions Fingers                                              64 Post Mechanical                                                              support                                                                   ______________________________________                                    

Although the present invention has been described with severalembodiments, various changes and modifications may be suggested to oneskilled in the art. Thus, it is intended that the present inventionencompass such changes and modifications as fall within the scope of theappended claims.

What is claimed is:
 1. A thermal sensor to provide an imagerepresentative of the amount of thermal radiation incident to thethermal sensor, comprising:a substrate having a surface; a thermallysensitive element spaced above the surface of the substrate; a firstthin film electrode including a plurality of spaced digits, the firstthin film electrode being provided on a side of the thermally sensitiveelement opposite from the substrate; a second thin film electrodeincluding a plurality of spaced digits, the second thin film electrodebeing provided on the same side of the thermally sensitive element asthe first thin film electrode, the digits of the first thin filmelectrode being in spaced interposed relation to the digits of thesecond thin film electrode; and a supporting structure to support thefirst and second thin film electrodes in the spaced relation above thesurface of the substrate, the supporting structure including:at leasttwo conductive posts extending upwardly from the surface of thesubstrate; and a pair of arms which each extend outwardly from arespective one of the electrodes and are each connected to a respectiveone of the posts.
 2. The thermal sensor of claim 1, wherein the spacebetween the thermally sensitive element and the surface of the substrateis approximately one-quarter of a wavelength of the thermal radiation tobe detected.
 3. The thermal sensor of claim 2, wherein the thermalsensor is free of material between the substrate and thermally sensitiveelement.
 4. The thermal sensor of claim 1, wherein the thermallysensitive element is a ferroelectric element.
 5. The thermal sensor ofclaim 1, wherein the digits of the first thin film electrode aresubstantially parallel to one another and the digits of the second thinfilm electrode are substantially parallel to one another.
 6. The thermalsensor of claim 1, further comprising:the digits of the first thin filmelectrode extending from a first end of the thermally sensitive elementtoward a second end of the thermally sensitive element; and the digitsof the second thin film electrode extending from the second end of thethermally sensitive element toward the first end of the thermallysensitive element.
 7. The thermal sensor of claim 1, wherein thethermally sensitive element is a pyroelectric material.
 8. The thermalsensor of claim 1, wherein the thermally sensitive element compriseslead lanthanum zirconate titanate (PLZT).